The good news is that Len, Ken and Ken all say they have experienced no antisemitism in the Labour party. Which must mean all is well. Surely only a pedant would point out that Ken Loach, Len McCluskey and Ken Livingstone are not Jewish – a fact that might limit their authority to speak on the matter.

Indeed, they have been fixtures on the left for so long – Loach is 81, Livingstone is 72 and McCluskey is 67 – perhaps they should sit as a panel. They could be the three wise men who arbitrate on all allegations of bigotry within Labour’s ranks. Then, if they testify that they have experienced no sexism, racism, Islamophobia or homophobia inside the party, we will know those menaces are blissfully absent from the prejudice-free nirvana that is the Labour family.

Labour fringe speaker's Holocaust remarks spark new antisemitism row

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More seriously, you would like to think that this trio, as longtime leftists, would have enough self-knowledge to recognise that, when it comes to, say, bias against women, black or LGBT people, straight, white men might not be best placed to judge. Yet, oddly, no such self-restraint seems to apply when it comes to anti-Jewish racism. Those who are not targeted suddenly feel fully entitled to tell those who are exactly what is – and what isn’t – prejudice against them.

Indeed, Len and Ken Loach go much further. They don’t just tell Jewish Labour supporters that they are mistaken to detect antisemitism around them: they tell them they have made it all up – and that they have done so for sinister, nefarious purposes.

“I believe it was mood music that was created by people who were trying to undermine Jeremy Corbyn,” McCluskey told BBC’s Newsnight. (Again, for an avowed progressive to describe an ethnic minority’s experience of racism as “mood music” is quite a break from the usual accepted practice.)

Loach expressed his scepticism differently. “It’s funny these stories suddenly appeared when Jeremy Corbyn became leader, isn’t it?”, the filmmaker told the BBC’s Daily Politics. But he was making the same point.

Meanwhile, Livingstone was on the radio cheerfully saying that it was perfectly possible to say offensive things about Jews without being anti-Jewish. He too has long argued that this whole business is bogus and confected, and that Labour does not have any kind of antisemitism problem.

And yet the evidence was there in Brighton if you were willing to see it. There were the Labour party Marxists handing out a paper that repeated Livingstone’s toxic claim of ideological solidarity between the Nazis and those German Jews who sought a Jewish homeland.

There were loud calls for the expulsion of Jewish groups, one of which has been part of the Labour movement for a century. Hardly a surprise that some Jewish activists turned away from the conference, describing an atmosphere that felt too hostile to endure.

But no – for Len and the Kens and their allies, it’s all made up. Perhaps they don’t realise that that itself is a tired anti-Jewish trope: that Jews invent stories of suffering to drive a secret political agenda. Or, to put it more simply, that there is a Jewish conspiracy.

It means that a man such as Ken Loach – an artist so sensitive he is capable of making the film I, Daniel Blake – ends up lending a spurious legitimacy to Holocaust denial. Asked to react to a speaker at a Brighton fringe meeting who had said Labour supporters should feel free to debate any topic, including the veracity of the Holocaust – “did it happen or didn’t it happen”, as the BBC interviewer put it – Loach could not give a simple, unequivocal denunciation of Holocaust denial. “I think history is for all of us to discuss,” he said.

Remember, Loach had not been asked whether there should be discussion of the meaning of the Nazi slaughter of the Jews. He had been asked about the fact of it happening. And on that, he said there should be discussion – the same apparently innocuous formulation routinely advanced by hardcore Holocaust deniers.

When distinguished men of the left are echoing, even inadvertently, the language of Holocaust denial, when the leader of Britain’s biggest trade union is rehashing the age-old notion of a Jewish conspiracy, you know you have entered a dark place. It’s not impossible to navigate your way out. But first you have to admit that you’ve got badly lost.